Rust and heat resisting ferrous base alloys containing chromium and aluminum

ABSTRACT

RUST AND HEAT RESISTING FERROUS BASE ALLOYS COMPRISING BY WEIGHT BETWEEN ABOUT 3% AND 10% ALUMINUM, BETWEEN ABOUT 4% AND 8% CHROMIUM, ABOUT 0.5% COPPER, LESS THAN 0.05% CARBON, AND THE BALANCE SUBSTANTIALLY IRON. THE ALUMINUM AND CHROMIUM TOGETHER TOTAL AT LEAST 9%, WITH EITHER THE CHROMIUM OR THE ALUMINUM BEING AT LEAST 6% AND THE DIFFERENCE BETWEEN THE CHROMIUM AND ALUMINUM BEING LESS THAN 6%, OF THE WEIGHT OF THE ALLOY. THE ALLOYS ACCORDING TO THIS INVENTION MAY BE RENDERED HEAT TREATABLE BY PRECIPITATION HARDENING BY ADDING BETWEEN ABOUT 0.5% AND 2.0% BY WEIGHT OF TITANIUM TO THE ALLOY TO PRODUCE RUST AND CORROSION RESISTANT ALLOYS OF HIGH TENSILE STRENGTH WITH GOOD DUCTILITY.

United'States Patent '01 3,676,109 Patented July 11, 1972 fice 3,676,109 RUST AND HEAT RESISTING FERROUS BASE ALLOYS CONTAINING CHROMIUM AND ALUMINUM Hugh S. Cooper, Shaker Heights, Ohio, assignor to Cooper Metallurgical Corporation, Cleveland, Ohio No Drawing. Filed Apr. 2, 1970, Ser. No. 25,278 Int. Cl. C22c 37/10, 39/14 US. Cl. 75-124 Claims ABSTRACT OF THE DISCLOSURE Rust and heat resisting ferrous base alloys comprising by weight between about 3% and aluminum, between about 4% and 8% chromium, about 0.5% copper, less than 0.05% carbon, and the balance substantially iron. The aluminum and chromium together total at least 9%, with either the chromium or the aluminum being at least 6% and the difference between the chromium and aluminum being less than 6%, of the weight of the alloy. The alloys according to this invention may be rendered heat treatable by precipitation hardening by adding between about 0.5% and 2.0% by weight of titanium to the alloy to produce rust and corrosion resistant alloys of high tensile strength with good ductility.

This invention relates to ferrous base alloys having stainless and heat resisting properties, but which contain relatively low amounts of chromium as compared to the usual stainless steels and contain aluminum and other alloying constituents in small amounts. Alloys according to this invention have heat and rust resistant properties comparable to Type 430 stainless steels, and are particularly useful in the construction of storm doors and windows and automobile trim. In these applications, the alloys must be competitive with aluminum on a cost basis and have comparable rust and scale resistant properties, while presenting a pleasing appearance. Therefore, the alloys should cost considerably less than the present cost of most stainless steels, and should approach the cost of ordinary low alloy steels.

The alloys according to this invention are also useful for the fabrication of certain chemical processing equipment subjected to corrosive and high-temperature environments, since the alloys are relatively inexpensive and are readily workable into useful shapes, and the alloy surface is relatively insensitive, thus reducing the amount of care required to work the ingot to sheet form while maintaining a stainless surface.

The usefulness of the corrosion resistant alloys of the invention may be extended into numerous other areas by adding a small amount of titanium and heat treating them to effect precipitation hardening. The yield and tensile strengths of such hardened alloys may be greatly increased, while retaining good ductility, by cold-rolling and aging above 1000 (3., Without scaling or loss of corrosion resistance, thus further extending their areas of usefulness.

According to this invention, the essential alloying ingredients are between about 3% and 10% aluminum, between about 4% and 8% chromium, about 0.5% copper, and less than .05 carbon, by weight, the balance of the alloy being essentially iron. It has been found that highly predictable stainless properties are obtained by using the aluminum and chromium alloying ingredients within the above-noted ranges when the total of these two elements is at least 9% and either the chromium or aluminum constitutes at least 6% of the alloy by weight. Furthermore, the difference between the amounts of chromium and aluminum contained in the alloy should be less than 6% of the weight of the alloy. Alloys having optimum corrosion resistant properties are obtained within the ranges set forth above when the total amount of aluminum and chromium is between 11% and 12% and the difference between their amounts is less than 1% of the weight of the alloy. The chromium in these alloys appears to have a mitigating effect on the embrittling tendency of aluminum. The low carbon content prevents carbide migration to grain boundaries which would promote corrosion. The copper in these alloys improves resistance to rust spotting.

A small amount of manganese, e.g., about 0.5%, may be used in these alloys as a deoxidizer according to common practice, and a small amount of boron, e.g., about less than .05 added as ferro boron may be found beneficial as a grain refiner.

Corrosion tests were conducted on the alloys set forth in the following tables. The alloy specimens were prepared by casting the alloys into ingots and then inspecting each ingot for defects by cutting a section from each ingot to examine it for piping and internal defects. The surfaces of each ingot were ground and, following the grinding operation, the surface was sanded on a l20-grit silicon carbide sanding belt until an even surface was obtained with a sanded matte finish. Atmospheric corrosion tests were conducted by mounting each specimen on a plywood panel by means of plastic pins, and the plywood panel was mounted on a pole, approximately ten feet off the ground, to eliminate localized splashing or ground interference. The panel was mounted at an angle of thirty degrees to the vertical, facing upwardly in a southerly direction. After 24 weeks, the specimens were inspected and were given a corrosion test designator ranging from zero to 4. The designator zero was intended for specimens which showed no apparent changes on the surface. The designator 1 was given to samples which showed very slight surface changes but which had no rust spotting or dark-colored staining. The designator 2 was given to samples showing only one or two very small rust spots or small areas of dark stain, but which were otherwise in a shiny condition. The designator 3 was given to samples which were spotted in more than two spots, or which showed staining of the entire surface or other evidence of corrosion over much of the sample. The designator 4 was given to samples which were badly corroded and rust-spotted over the entire surface.

TABLE I Percent Corrosion Alloy Aluminum Chromium Copper Iron designator 6 5 0.5 Balance- 1-2 6 6 0.5 1-2 5 6 0.5 1-2 10 5 0.5 2 9 4 0.5 2 6 4 0.5 2 4 6 0.5 2 3 6 0.5 2 3 7 0.5 2 4 7 0.5 2

All of the alloys set forth in Table I contain between about 3% and 10% aluminum, between about 4% and 8% chromium, about 0.5% copper, less than .05% carbon, and the balance essentially iron. The alloys in Table I also have at least 9% total aluminum and chromium, with either the chromium or aluminum being at least 6% and the difference between the percentages of the chromi um and aluminum being less than 6. It should be noted that alloys 1, 2, and 3 have the best corrosion resistant properties, and those three alloys all have between about 11% and 12% total aluminum and chromium, with the difference in the percentages of chromium and aluminum being from zero to 1.

test designator of 4.

TABLE II Percent Corrosion Alloy Aluminum Chromium Copper Iron designa r 11 5 0.5 Balance. 4 12 5 4 0.5 do 4 The alloys of Table 111 below contain between about 3% and aluminum, between about 4% and 8% chromium, 0.5% copper, less than .05 carbon, and the balance essentially iron. Although the difference between the chromium and aluminum percentages in these alloys is less than 6, the total chromium and aluminum is less than 9% and neither the chromium nor aluminum percentage is as great as 6%. Alloy No. 13 corroded badly and was given a corrosion test designator of 4, and alloy No. 14 was rust-spotted at more than three spots and was therefore given a corrosion test designator of 3.

TABLE III Percent Corrosion Alloy Aluminum Chromium Copper Iron designa t b r 13 4 4 0.5 Balance. 4 14 3 4 0.5 do 3 The alloys of Table IV below have differences between the chromium and aluminum percentages of less than 6, but all had insufficient chromium, aluminum, or both, to satisfy the parameters of the invention. All corroded rather badly, only one deserving a designator as low as 3.

TABLE IV Percent Corrosion test Chromium Copper Iron designator O. 5 Balance. do.

NWUWWNOINN-HH e calatoran mine ra-pwimms-cmamww- TABLE V Percent Corrosion test Alloy Aluminum Chromium Copper Iron designator 26 2 6 0.5 Balance- 4 27- 1 6 0.5 .(10.... 4

An alloy having a total amount of chromium and aluminum well above 9% and having an aluminum content greater than 6%, but having only a small deficiency of chromium according to the parameters of the invention, is set forth in Table VI. As indicated, it corroded badly.

4 Table VI Alloy 28 Aluminum, percent 9 Chromium, percent 3 Copper, percent 0.5 Iron, percent Balance Corrosion text designator 4 It has also been found that alloys according to this invention may be rendered heat treatable by precipitation hardening by the addition of between about 0.5 and 2% titanium. For example, an alloy having 6.7% aluminum, 6.5% chromium, .61% copper, .01% carbon, .78% titanium, and the balance iron, was cast into an ingot, hotrolled, and heated to 1650 F. for one-half hour. A sample cut from the ingot exhibited a yield strength (0.2% offset) of 57,500 p.s.i. and a tensile strength of 76,400 p.s.i., with a 23% elongation (two-inch gauge). After cold-rolling with a 50% reduction and aging at 1025 F. for four hour, the sample had a yield strength (0.2% olfset) of 144,600 p.s.i., a tensile strength of 156,200 p.s.i., and 8% elongation (two-inch gauge). This sample, when exposed to oxidizing atmospheres at a temperature of about 1800" F., exhibited minimum scaling. Thus, an important aspect of the invention involves the production of heat treatable, rust and heat resistant alloys by such titanium additions.

The invention is not restricted to the slavish imitation of each and every one of the details set forth above. Obviously, processes may be provided which change, eliminate, or add certain details without departing from the scope of the present invention.

What is claimed is:

1. A rust and heat resisting ferrous base alloy consisting essentially of, by weight, between about 3% and 10% aluminum, between about 4% and 8% chromium, about 0.5% copper, less than .05 carbon, and the balance essentially iron, said chromium and aluminum totalling at least 9%, either said chromium or aluminum being present in an amount of at least 6%, and the difference bebeing less than 6.

2. A rust and heat resisting ferrous base alloy according to claim 1, wherein said chromium and aluminum total between about 11% and 12% and wherein the difference between the percentages of said chromium and aluminum is 1 or less.

3. A heat treatable rust and heat resisting ferrous base alloy consisting essentially of, by weight, between about 3% and 10% aluminum, between about 4% and 8% chromium, about 0.5% copper, less than .05% carbon, between about 0.5% and 2% titanium, and the balance essentially iron, said chromium and aluminum totalling at least 9%, either said chromium or aluminum being present in an amount of at least 6%, the difference between the percentages of said chromium and aluminum being less than 6.

4. A heat treatable rust and heat resisting ferrous base alloy according to claim 3, wherein said titanium is about .78%.

5. A heat treatable rust and heat resisting ferrous base alloy consisting essentially of about 6.5% chromium, 6.7% aluminum, .61% copper, .01% carbon, .78% titanium, and the balance essentially iron.

References Cited UNITED STATES PATENTS 1,710,805 4/1929 Smith 75-124 1,759,606 5/1'930 De Vries 75125 1,763,421 6/ 1930 De Vries 75--124 HYLAND BIZOT, Primary Examiner US. Cl. X.R. 75--l26 R 

